diff --git a/include/spdlog/details/os-inl.h b/include/spdlog/details/os-inl.h
index 19d4bdc5..46e985a4 100644
--- a/include/spdlog/details/os-inl.h
+++ b/include/spdlog/details/os-inl.h
@@ -18,6 +18,7 @@
 #include <string>
 #include <thread>
 #include <array>
+#include <cassert>
 #include <sys/stat.h>
 #include <sys/types.h>
 
@@ -501,20 +502,16 @@ SPDLOG_INLINE void utf8_to_wstrbuf(string_view_t str, wmemory_buf_t &target)
         return;
     }
 
-    int result_size = static_cast<int>(target.capacity());
-    if (str_size + 1 > result_size)
-    {
-        result_size = ::MultiByteToWideChar(CP_UTF8, MB_ERR_INVALID_CHARS, str.data(), str_size, NULL, 0);
-    }
+    // find the size to allocate for the result buffer
+    int result_size = ::MultiByteToWideChar(CP_UTF8, MB_ERR_INVALID_CHARS, str.data(), str_size, NULL, 0);
 
     if (result_size > 0)
     {
         target.resize(result_size);
         result_size = ::MultiByteToWideChar(CP_UTF8, MB_ERR_INVALID_CHARS, str.data(), str_size, target.data(), result_size);
-
-        if (result_size > 0)
+        if (result_size  > 0)
         {
-            target.resize(result_size);
+            assert result_size == target.size();
             return;
         }
     }